Environmental Engineering Reference
In-Depth Information
At this early stage, the final structure of any hydrogen economy is unclear and
it remains uncertain how it should be best integrated with the existing electrical
infrastructure. One of the limiting factors in future development is fuel cell effi-
ciency - a likely target figure is 60 per cent. If this is combined with an electrolyser
efficiency of 90 per cent and a compression storage efficiency of 90 per cent, then
an overall cycle efficiency of about 50 per cent is achievable. This compares
unfavourably with cycle efficiency figures for pumped storage (60-80 per cent),
secondary batteries (75-85 per cent), flow batteries (75-80 per cent) and com-
pressed air storage (75 per cent). The one advantage offered by hydrogen not
available to the other technologies is the ease with which energy generation and
storage can be physically separated, assuming that a pipeline/transport infra-
structure is in place. The low efficiency ( 20 per cent) of today's internal com-
bustion engines and their noxious emissions strongly suggest, however, that fuel
cell vehicles have a brighter future.
A number of options for system integration are available: hydrogen could be
stored at a utility level and then distributed electrically along transmission lines
during peak demand periods; hydrogen could be pumped by pipeline to widespread
locations and then transmitted electrically along distribution lines; or hydrogen
could be delivered direct to the end consumer where a fuel cell would provide
electricity and (waste) space heating. Alternatively, the electrolysed hydrogen
could be used to drive a fleet of hydrogen-powered cars, buses and other vehicles.
In all likelihood, a combination of the above scenarios will evolve, with govern-
ment policy and topographical issues as well as technical and economic arguments
influencing the final balance (Anderson and Leach, 2004).
Wind generation can fit into this proposed hierarchy in a number of ways: low
scale, on-site hydrogen production would mitigate wind generation variability,
protect against regional network overloading and/or provide a short-term back up
supply during becalmed periods. Scottish Power, for example, proposed exploiting
excess wind output (which cannot be accepted on to local grids in Scotland) for
hydrogen production. Alternatively, wind farms may be built with the primary
objective of providing hydrogen rather than utility connected electricity. Any such
exploitation of hydrogen for transport or other applications (rather than fuel cell
generation) will lessen the power system, if not commercial, benefit. With the
expected growth of large offshore wind farms, a hydrogen pipeline offers an
alternative method of transporting wind energy ashore. The necessary equipment
costs and transportation losses are comparable with laying a high voltage cable.
However, given the low cycle efficiency of converting electricity to hydrogen and
back again, this can only be justified if there is a benefit to the system or a market
exists for the hydrogen supply.
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